PHP class and setting properties

So I ran into a problem while building a class in which I was unable to set the property of the class directly, and instead had to set it during construction. Here is an example of what I was trying to do.

class foo
{
    private $con = Db::init();

    public function __construct()
    {
    }

    //continue class..
}

As you can see, I am just assigning a simple singleton PDO class to the property. This does not work, and I am forced to do the following.

class foo
{
    private $con;

    public function __construct()
    {
        $this->con = Db::init();
    }

    //continue class..
}

The开发者_如何学编程 first approach does not report any errors either. It just fails to continue execution. Any thoughts?

edit

The lack of errors may also be a Zen Cart thing.

What is happening here is the class is a structure, and the structure is compiled before your PHP Fiel is executed, as in the compile time PHP Does not instantiate any dynamic data, you cannot use dynamic data.

For example:

$function = 'hello';
function $function(){}

Within the compile time the variable '$function' does not exists so it cannot be read, within your class PHP Has provided a function called __construct which is fired within run-time, meaning that the rest of the system's dynamic data is available.

class foo
{
    protected $bar;

    public function __construct()
    {
        $this->bar = Db::Init();
    }
}

So the process is:

• Compile Time
  • Class foo
  • variable bar
  • function __construct
• Run Time
  • new foo found
  • internally create object
  • execute foo::__construct()
  • return foo

That's a simplified version of the process, there are some several ways to set objects to a class, you can do the regular approach as shown above

you can inject by doing:

public function __construct(Bar $bar)
{
    $this->bar = $bar;
}

you can create a base class and extend:

class DatabaseAccess
{
    protected $db;

    public function __construct()
    {
        $this->db = Db::Init();
    }
}

and then do:

class User extends DatabaseAccess
{
    public function getUser($id)
    {
         $this->db->fetchRow('users',$id);
    }
}

}

Please see the class property docs at http://www.php.net/manual/en/language.oop5.properties.php

... this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

Db::init() must be evaluated at run time to determine the value to set the private $con, which conflicts with the compile time requirement of class properties.